1. Field of the Invention
The present invention relates generally to cathode ray tube (CRT) displays and particularly to circuitry and methodology for determining location of a light pen on the face of a CRT.
2. Description of the Prior Art
Light pens are generally hand-held, light detecting devices which allow a human operator to initiate and/or perform various data processing operations or manipulations by placing the sensing aperture of the light pen adjacent to an appropriate location on the face of its associated CRT. The image on a CRT is generated by using an electron beam to stimulate selected areas of a phosphorescent material located on the inside of the CRT screen. The scanning of the CRT face is accomplished by deflecting the electron beam relatively rapidly in one direction, usually horizontal, and relatively slowly in a second direction, usually vertical. The phosphorescent material on the screen is continuous, but the screen can be considered to consist of a large number of generally horizontal, parallel "raster lines" or lines of displayed information. As the beam scans along a raster line, the information about the level of stimulation to be given a particular area on the raster line is updated at fixed intervals in accordance with a clock pulse from the CRT timing unit. Therefore, each raster line can be further considered to be a series of discrete segments or "dots" which are individually stimulatable by the electron beam.
The electron beam normally performs 50 to 60 "frames" or complete scans of the CRT screen per second, depending on the external electrical power available. From the viewpoint of an observer facing the screen, the beam begins a frame at the left side of the top raster line of the CRT and moves horizontally along the line to the right side of the screen stimulating each dot to the appropriate level to create the desired image. The beam then performs a horizontal retrace to the left side of the next lower raster line and again begins to scan horizontally to the right. This continues until the beam reaches the right side of the lowest raster line, at which time a vertical retrace is performed during which the beam moves back to the beginning of the top raster line to begin the next frame.
When the sensing aperture of the light pen is placed against or in proximity to the CRT face, it can sense the beam passing its "field of view" during each scan cycle. The processing unit associated with the CRT and light pen can, therefore, determine precise location of the pen on the CRT face by utilizing information about when the beam was detected by the pen.
Accurate, repeatable determination of light pen location, however, has been a long standing problem in industry. Prior art techniques for identifying the site of the light pen suffer from a substantial amount of noise and jitter. A cursor is commonly displayed on the CRT screen to indicate to the operator where the processor has determined the light pen to be. In prior art devices the cursor often exhibits jitter in that it is very difficult for the operator to place and/or hold the cursor in a desired location or to move the cursor along a desired path. These problems serve to substantially reduce the effective definition capability of the screen.
The present invention relates to a novel light pen detection circuit and method which allow the cursor generated by the CRT to exhibit greatly reduced jitter and noise.